On the General page of the Properties view, type TotalPopulation in the Initial Value edit box. Use intelli-sense mechanism to avoid typing the whole names of variables and functions. To open the intelli-sense wizard, click at the desired position in the edit box, and then press Ctrl+space.
The wizard listing model variables and functions appears. Scroll to the name you want to add, or type the first letters of the name until it becomes visible in the list. Finally, press Enter to insert the name.

We do not need to set up the initial value for the Adopters stock explicitly since there are no adopters initially and the stock is already initialized with zero by default.